There is known a conventional thermal head of a plane type in which a driver IC is provided on one portion of a main plane surface of the substrate of the thermal head and a heating resistor is provided at another portion of the main plane surface.
There is known another conventional thermal head of a subplane type in which merely a heating resistor is provided on a portion of a subplane surface of the substrate of the thermal head whereas a driver IC is not provided on any portion of the subplane surface. As shown in FIG. 1, a substrate 1 made of alumina or the like has a main plane surface la on which the driver IC not shown in FIG. 1 is provided, and a subplane surface 1b on which a heating resistor 3 is provided and which extends perpendicularly to the main plane surface. A glaze 2 is bonded by firing to the main and subplane surfaces 1a and 1b of the substrate 1 in order to keep the surfaces flat and smooth. As disclosed in the Laid-open Japanese Patent Application (OPI) No. 24695/85, the thermal head has various advantages such as that the heating resistor 3 comes into good contact with thermosensitive paper, a relief means for preventing the driver IC from coming into contact with a platen does not need to be provided, it is therefore easy to make the whole thermal head compact, and it is easy to secure the flatness of the portion on which the heating resistor 3 is provided.
The thickness of the glaze 2 on an edge 1c thereof between the main plane surface 1a and subplane surface 1b of the substrate 1 tends to be smaller than that of the glaze on the main plane and the subplane surfaces due to the surface tension of the glaze as the glaze is bonded by firing to the plane surfaces. If the thickness of the glaze 2 on the edge 1c is set at a proper value, that of the glaze on the main plane and the subplane surfaces 1a and 1b becomes larger than the proper value so as to deteriorate a heat transmitting property of the thermal head to lower a printing speed thereof. On the other hand, if the thickness of the glaze 2 on the main plane and the subplane surfaces 1a and 1b is set at the proper value, that of the glaze on the edge 1c becomes smaller than the proper value so as to lower the flatness and smoothness of the glaze on the edge thereof to disconnect a wiring pattern of the thermal head. This is a problem. Further, since the area where the heating resistor 3 can be provided extends in the direction of the thickness of the substrate 1, the degree of freedom of a combination of the area and the thickness is extremely limited. This is another problem.
Although the substrate of yet another conventional thermal head is provided with a sloped surface between the main plane and subplane surfaces of the substrate by chamfering the edge between the plane surfaces or by the like and a glaze and a heating resistor are then sequentially provided on the sloped surface as disclosed in the Laid-open Japanese Utility Model Application (OPI) No. 13333/89, the glaze is not provided on the subplane surface. For that reason, the wiring pattern of the thermal head is likely to be disconnected. Further, the thickness of the glaze would not be controlled in the Application.